1. Field of the Invention
This invention relates to apparatus for measuring voltages on bus bars and in particular to capacitive divider type voltage measuring apparatus which utilizes the bus bar as one plate, and layers of electrical insulation alternating with conductive layers to form the dielectrics and other plates for the capacitors of the capacitive divider. It also relates to such voltage measuring apparatus in combination with current measuring apparatus which utilizes the conductive layers as a voltage shield.
2. Background Information
Conventional apparatus for measuring current and voltage on medium to high voltage bus bars includes current and potential transformers providing signals to 120 volt 5 amp electromechanical devices which generate indications of the measured current and voltage. For very high voltages, where the cost of a potential transformer would be prohibitive, capacitive dividers are used to measure the voltage to ground. In such a device, a first capacitor with a very small capacitance is connected to the bus bar. A second capacitor with a much larger capacitance is then connected in series between the first capacitor and ground. The voltage across the second capacitor is proportional to and is taken as a measure of the voltage between the bus bar and ground.
Such capacitive dividers have not been used to measure other than the very high voltages because they are not compatible for use with the 120v, 5 amp electromechanical metering and protection devices which have been standard for lower voltages Particularly, the capacitive divider cannot be used with a measuring device which places a load on the divider.
Recent developments include solid state circuitry for measuring current and voltage which operates with low voltage and has a very high input impedance. Current sensing is achieved with mutual inductors which generate a signal proportional to the derivative of the sensed current. This signal is applied to an integrator which generates a signal proportional to the measured current These mutual inductors have an air core or a very low permeability powdered iron core. Hence, the signal level is very low. Shielding is provided to isolate the mutual inductor from the electric field produced by the voltage on the bus bar. This shielding includes wrapping the mutual inductor in a non-magnetic conductor such as copper foil. Alternatively, conductive paint is applied over the insulation covering the bus bar in the vicinity of the mutual inductor and grounded to form the shield. Typically, these current and voltage sensing devices are utilized with apparatus such as meters, contactors and circuit interrupters where space is limited.
There remains a need, however, for a low cost, simple, yet reliable device for measuring voltage on the bus bar.
There is a related need for such a device which can take advantage of other recent developments in the art of current and voltage measurement and can easily be retrofit to devices incorporating these developments.
More particularly, there is an additional need for such a device which can be combined with and take advantage of the improvements in current sensors.
There is also a need that such a device be compact so that it can be used in the confines of medium voltage apparatus having a requirement for voltage and current measurements.